1. Field of the Invention
The present invention relates to a concentration measuring apparatus for hydrogen sulfide in a gas flow and a method for deter mining sulfide ion.
2. Related Background Art
Hydrogen sulfide (H2S) is a poisonous and highly corrosive gas. Many working environments are accompanied by the risk of poisoning by H2S. A human nose may sense H2S from a concentration of H2S as low as approximately 0.02 ppm, and the maximum sensitivity is usually approximately 5 ppm. Because the olfactory nerve begins to be paralyzed under the influence of a neurotoxin by H2S with a higher concentration (or exposure to H2S for a long time), the sensing ability of the gas is reduced (for example, see Literature 1 (P. Patnaik, “A ComprehensiveGuide to the Hazardous Properties of Chemical Substances”, second ed., Wiley, New York, N.Y., 1999.) and Literature 2 (W. Puacz, W. Szahun, Analyst 120, (1995) 939.)).
Contribution of hydrogen sulfide to the total sulfur content in an environment (the air) is relatively limited. Emission of H2S to the environment (the air) is mainly attributed to industrial areas (particularly petrochemical industry) concentrated on a specific place (for example, see Literature 3 (W. L. Orr, J. S. “SinningheDamste, Geochemistry of sulfur in petroleum systems”, in: W. L. Orr, C. M. White (Eds.), “Geochemistry of Sulfur in Fossil Fuels”, ACS Symposium Series 429, American Chemical Society, Washington, D.C., 1990, pp. 2-9.”), Literature 4 (A. C. Aplin, M. L. Coleman, “Sour gas and water chemistry of the Bridgeport Sands reservoir”, Wytch Farm, UK, in: J. M. Cubitt, W. A. England (Eds.), “The Geochemistry of Reservoirs”, The Geological Society, London, UK, 1995, pp. 303-314 Geol Soc. Special Publication 86.), and Literature 5 (R. D. Kane, R. J. Horvath, M. S. Cayard (Eds.), “Wet H2S cracking of carbon steels and weldments”, NACE International, Houston, 1996.)). Hydrogen sulfide exists widely in a hydrocarbon storage layer under the ground under an anaerobic condition as a result of interaction of sulfate-reducing bacteria and exogenous sulfate (see the above Literatures 3 and 4, for example). Generally, the proportion of sulfur contained in crude oil is in the range of 0.3 to 0.8 wt %, and the proportion of hydrogen sulfide contained in natural gas is in the range of 0.01 to 0.4 wt %. While it has been reported that the concentration of hydrogen sulfide in the natural gas is 30 wt % at the maximum (for example, see Literature 6 (Dosher, J. R and Carney J. T., “Sulfur increase seen mostly in heavy fractions of lower-quality crudes” Oil & Gas J., 92, 42-48 (1994).), it is recognized that the proportion of sulfur contained in the crude oil and that in the natural gas have been increased steadily for the past decades, and further increase in the concentration of hydrogen sulfide is expected (for example, see Literature 7 (M. R. Carlson, W. B. Cawston, “Obtaining PVT data for very sour retrograde condensate gas and volatile oil reservoirs: A multi-disciplinary approach”, SPE Gas Technology Conference, Calgary, Canada, April-May 1996, SPE 35653.)).
H2S is emitted as a by-product of biological action in recovery and processing of waste water. Although H2S considerably dissolves in water, most of bad smell problems caused by the waste disposal process are attributed to temporary emission of H2S. In addition, sulfides are emitted to an aqueous environment through mobilization of sulfur containing inorganic substances by microorganisms (for example, see Literature 8 (B. Meyer, “Sulfur Energy and the Environment”, Elsevier, 1977.)). Leather article industry (for example, see Literature 9 (J. Font, J. Gutierrez, J. Lalueza, X. Perez, J. Chromatogr. A 740 (1996) 125.) and pulp and paper industry (for example, see Literature 10 (D. R. Saloman, J. Romano, J. Chromatogr. 602 (1992) 219.)) also greatly contribute to emission of H2S or S2− to an ecological system.
Because sulfide anions have high reactivity, development of many detecting methods ranging from more classic methods to span spectroscopic, chromatographic, electrochemical methods, and their combinations thereof have been possible. Reviews (surveys) including some of these methods have been reported (for example, see Literature 11 (N. S. Lawrence, J. Davis, R. G. Compton, Talanta 52 (2000) 771-784.) and Literature 12 (L. Ferrer, M. Miro, J. M. Estela, V. Cerda, Trends in Analytical Chemistry, 26 (2007) 413-422.)). However, most of the methods developed so far aim at measurement of sulfide anions in a solution, and the methods designed in order to measure H2S in a gas flow were few. As the methods designed in order to measure H2S in a gas flow, solid state gas sensors and gas analyzers based on ultraviolet absorption or a lead acetate tape method are known. As a detector (solid state gas sensor) using a semiconductor, those disclosed in the following Patent Literature 1 (U.S. Pat. No. 3,479,257), Patent Literature 2 (U.S. Pat. No. 3,901,067), Patent Literature 3 (U.S. Pat. No. 4,030,340) and Patent Literature 4 (U.S. Pat. No. 3,567,383) are known, for example. As an H2S analyzer based on ultraviolet absorption measurement in a flow cell (gas analyzer based on ultraviolet absorption), that disclosed in the following Non Patent Literature 1 (A. J. Rollo, “Diode ArrayProcess Analyzer—for Sulfur Recovery Applications”, Instrumentation, Systems, and Automation Society, ISA 52nd Analysis Division Symposium, 2007) is known, for example.
The detector (solid state gas sensor) using a semiconductor utilizes a fact that a specific substance adsorbed by the semiconductor influences conductivity of a thin portion (segment) of a thin film in the vicinity of the surface of the semiconductor. These apparatuses are usually formed of a metal oxide semiconductor provided on an inactive substrate. For giving an influence to conductivity, a small amount of a dopant having a valence higher or lower than that of a metal oxide, namely, impurities may be added to the metal oxide. Further, in order to accelerate a reaction in a surrounding gas and to assist detection, a catalyst may be added to the surface of the semiconductor. These various apparatuses are disclosed in Patent Literatures 1 to 4 above, for example. These apparatuses and other apparatuses are subjected to one or more restrictions. For example, many of such apparatuses cannot provide sufficient life span. Other apparatuses depend on a catalyst for decomposing the gas to be detected. However, because the catalyst tends to be influenced by a catalyst poison, the life span of the apparatus is limited. Other apparatuses are greatly affected by moisture (humidity) in the gas. For that reason, it is difficult to sufficiently enhance reliability of the apparatus.
The lead acetate tape method for detecting hydrogen sulfide (H2S) and total sulfur in a gaseous flow is based on an established principle that H2S reacts specifically with lead acetate to produce a brown lead sulfide pigment. The concentration of H2S is in direct proportion to a changing speed of coloring of a lead acetate tape. This principle serves as a basic principle of many ASTM (American Society for Testing and Materials) methods. This analyzer moves a processed paper tape by one partition at one time. According to the concentration of a sample, the color of the tape begins to be darken at a rate proportional to the concentration of H2S in the flow of the sample. This analyzer exposes a new partition of the tape to the sample in a sample chamber for every specific time (for example, every three minutes). Although this tape analyzer is reliable and is thought as a simple method, the tape analyzer can be used only for semi-continuous measurement because the reaction with a slow reaction rate is utilized.
Although the H2S analyzer based on ultraviolet absorption measurement in a flow cell can continuously measure H2S in a gas flow, the H2S analyzer cannot be available without considerably high cost, because it includes expensive components such as a xenon pulse light source and a diode array detector (see Non Patent Literature 1 above).
The present invention has been made in consideration of such problems; an object of the present invention is to provide a concentration measuring apparatus for hydrogen sulfide that can continuously measure hydrogen sulfide in a gas flow, and is inexpensive and reliable; and another object is to provide a method for determining sulfide ion in a liquid that can be used for such a concentration measuring apparatus for hydrogen sulfide.